In recent years, an improvement in fuel consumption performance of vehicles, such as automobiles, etc., is a big problem, and as one means for solving that problem, an improvement in fuel consumption reducing properties of a lubricating oil composition is demanded.
As a measure for improving the fuel consumption reducing properties of a lubricating oil composition, the development of a lubricating oil composition that may reduce friction between mechanical parts or a lubricating oil composition having reduced viscous resistance is performed. As for an additive capable of reducing the viscous resistance of a lubricating oil composition, the development and selection of a viscosity index improver are widely performed.
From the viewpoint of an improvement in fuel consumption reducing properties of a lubricating oil composition, for example, polymethacrylate-based viscosity index improvers have hitherto been used for engine oil.
For example, PTL 1 discloses a lubricating oil composition for internal combustion engine, in which an ashless dispersant, a polymethacrylate-based viscosity index improver whose PSSI (permanent shear stability index) falls within a predetermined range, and the like are contained in a lubricating base oil, and a ratio of a viscosity index to an HTHS viscosity (high temperature high shear viscosity) at 100° C. is regulated to a predetermined range.
It is mentioned that the lubricating oil composition for internal combustion engine described in PTL 1 is good in fuel consumption reducing properties in a high-temperature region.
In addition, PTL 2 discloses a lubricating oil composition for internal combustion engine, in which a comb-shaped polymer having a specified weight average molecular weight and including a repeating unit based on a polyolefin macromonomer, a repeating unit based on an alkyl (meth)acrylate, and a repeating unit based on a styrene-based monomer in a main chain thereof is contained as a viscosity index improver in a base oil, the lubricating oil composition having a predetermined viscosity index.
It is mentioned that the lubricating oil composition for internal combustion engine described in PTL 2 has excellent coking resistance and shear stability while having a high viscosity index.